If you do vertical panels on a van, depending on the van, you will have to match up 4 or more seams on each side. If you do horizontal panels on a van, you just have one seam. Place that one seam down low and it will be out of most people's line of sight when the are looking at the van. It won't be as noticeable.
You're right, it is a matter of opinion. My opinion is that vertical panels look like ass on just about anything... especially when there is a way to do it with fewer seams or no seams at all.